A groundbreaking new study examines the origins of holiday giving and finds that our early human ancestors were frequently altruistic.

"Reciprocity is arguably the foundational basis of cooperation in humans," writes Michael Gurven (University of California – Santa Barbara). "A core feature of reciprocity is the contingent relationship between acts of giving and receiving among social partners. Contingency is important because it sets the rules for who qualifies as a free-rider or cheater in exchange relations."

A rigorous effort to quantify the extent and magnitude of different forms of payback in exchange relationships, the study, forthcoming in the February 2006 issue of Current Anthropology, will be critical for resolving heated debates about the function of altruism among hunter-gatherers.

Strict forms of contingency require tit-for-tat, while more forgiving forms emphasize the work effort or relative contributions of others. Gurven examined food exchanges in two small-scale, non-market societies – a classic context for understanding the evolution of conditional cooperation in humans.

"Without some kind of payback, altruism can be a very costly endeavor in small-scale societies subsisting on wild foods," Gurven writes. "This study shows that people indeed share more with those who give more to them… [but] families who cannot produce much food, close kin, and nearby neighbors sometimes receive more than they give."